Crude petroleum is distilled and fractionated into many products such as gasoline, kerosene, jet fuel, vacuum gas oil (VGO) and deashphalted oil (DAO), and the like. One portion of the crude petroleum forms the base of lubricating base oils used in, inter alia, the lubricating of internal combustion engines. Lube oil users are demanding ever increasing base oil quality, and refiners are finding that they need to use hydroprocessing to produce base oils that meet these higher quality specifications. New processes and higher severity are required to provide refiners with the tools for preparing high quality modern base oils, particularly using existing equipment at lower cost and with safer operation.
Finished lubricants used for such things as automobiles, diesel engines, and industrial applications generally are comprised of a lube base oil and additives. In general, a few lube base oils are used to produce a wide variety of finished lubricants by varying the mixtures of individual lube base oils of different viscosity grades and individual additives. Typically, lube base oils are simply hydrocarbons prepared from petroleum or other sources. Lube base oils are normally manufactured by making narrow cuts of vacuum gas oils from a crude vacuum tower. The cut points are set to control the final viscosity and volatility of the lube base oil.
In the prior art, Group I base oils, those with greater than 300 ppm sulfur and 10 wt. % aromatics have been generally produced by first extracting a vacuum gas oil (or waxy distillate) with a polar solvent, such as N-methyl-pyrrolidone, furfural, or phenol. The resulting waxy raffinates produced from solvent extraction process are then dewaxed, either catalytically with the use of a dewaxing catalyst such as ZSM-5, or by solvent dewaxing to improve cold flow property like pour point. The resultant base oil may be hydrofinished to improve color and other lubricant properties.
Group II base oils, those with less than 300 ppm sulfur and saturates greater than 90%, and with a viscosity index range of 80-120, have been typically produced by hydrocracking followed by selective catalytic dewaxing and hydrofinishing. Hydrocracking upgrades the viscosity index of the entrained oil in the feedstock by ring opening and aromatics saturation. The degree of aromatics saturation is thermodynamic equilibrium limited reaction, thus extent of reaction is limited by hydrogen partial pressure and reaction temperature in hydrocracking stage. In the down stream process, the hydrocracked oil is dewaxed, either by solvent dewaxing or by catalytic dewaxing, with catalytic dewaxing typically being the preferred using hydroisomerization dewaxing technology. The dewaxed oil is then preferably hydrofinished at mild temperatures to remove trace olefins and polynuclear aromatics which were may be formed due to acidic nature of in the hydrocracking/dewaxing stage and which have a strongly detrimental impact on lube base oil quality.
Group III base oils have the same sulfur and aromatics specifications as Group II base stocks but require viscosity indices above 120. These materials have been manufactured with the same type of catalytic technology employed to produce Group II base oils but with either the hydrocracker being operated at much higher severity, or with the use of very waxy feedstocks.
A typical lube hydroprocessing plant known in the prior art consists of two primary processing stages. In the lead stage, a feedstock, typically a vacuum gas oil, deasphalted oil, processed gas oils, or any combination of these materials, is hydrocracked or solvent extracted. The hydrocracking stage upgrades the viscosity index of the entrained oil in the feedstock by ring opening and aromatics saturation. The degree of aromatics saturation is limited by the high temperature and hydrogen partial pressure of the hydrocracking stage. In a second stage, the hydrocracked oil is dewaxed, preferably with the use of a highly shape-selective catalyst capable of wax conversion by isomerization. The dewaxed oil can be subsequently hydrofinished at mild temperatures to remove polynuclear aromatics (PNAs) that were not converted in the upstream hydrocracking and dewaxing stages and which have a strongly detrimental impact on lube base oil quality. Operation of the final hydrofinishing step is optimized to convert polynuclear aromatics; conversion of these species and significant conversion of one ring and two ring aromatics cannot be accomplished in the final hydrofinishing step because of its low operating temperature.
In the prior art, PNAs have been removed primarily by a noble metal catalytic process that converts the PNAs to non-aromatic compounds via saturation and cracking, for example. However, PNAs are known to cause deactivation of these catalysts, thus requiring frequent replacement and/or regeneration of the catalyst. Regeneration of the catalyst reduces process yield, increases process costs, and increases the manufacturing time for the lube oils to be manufactured.
Accordingly, it is desirable to provide improved systems and methods for the manufacture of high quality lube oils. Additionally, it is desirable to provide such methods and systems that eliminate or substantially reduces the presence of PNAs so that down stream processes do not deactivate prematurely. Furthermore, other desirable features and characteristics of the present disclosure will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the drawings and this background.